1. Technical Field
The present disclosure relates to a testing head for a test equipment of electronic devices.
In particular but not exclusively, the disclosure relates to a testing head for a test equipment of electronic devices integrated on a wafer and the following description is made with reference to this field of application only for simplifying the description thereof.
2. Description of the Related Art
As is known, a testing head is a device able to put a plurality of contact pads of a microstructure, in particular an electronic device integrated on a wafer, into electrical connection with corresponding channels of a testing machine that performs the functionality check thereof, in particular an electrical check, or generically the testing.
The test run on integrated devices serves, in particular, for detecting and insulating faulty devices yet in the production step. Normally, testing heads are therefore used for the electrical testing of the devices integrated on a wafer before cutting and assembling the same within a package for containing chips.
A testing head comprises a plurality of mobile contact elements or contact probes retained by at least one pair of plates or guides substantially plate-shaped and parallel to one another. Such guides are provided with suitable holes and placed at a certain distance from each other so as to leave a free zone or air zone for the movement and any deformation of the contact probes. The pair of guides, in particular, comprises an upper guide and a lower guide, both provided with guide holes within which the contact probes, normally formed by wires of special alloys with good electrical and mechanical properties, axially slide.
The proper connection between the testing probes and the contact pads of the device being tested is ensured by the pressure of the testing head onto the device itself, the contact probes, mobile within the guide holes made into the upper and lower guides, undergoing a bending upon such pressing contact, within the air zone between the two guides and a sliding within such guide holes. Testing heads of this type are commonly called “vertical probes”.
The contact probes have, in particular, a contact tip that abuts onto corresponding contact pads of a device to be tested, performing the mechanical and electrical contact between said device and a test equipment whereof this testing head forms an end element. Usually, the contact probes have a further contact tip (also called contact head) towards a plurality of contact pads of a micro contact structure, also called space transformer.
A testing head of this type is schematically shown in FIG. 1, globally indicated with 1. The testing head 1 comprises at least one upper guide [upper die] 2 and one lower guide [lower die] 3, having respective upper 4 and lower 5 through guide holes wherein at least one contact probe 6 is slidingly engaged.
The contact probe 6 has a contact end or tip 7. In particular, the contact tip 7 is in mechanical contact with a contact pad 8 of a device 9 to be tested, while performing the electrical contact between said device 9 and a test equipment (not shown) whereof such testing head constitutes an end element.
The upper 2 and lower 3 guides are suitably spaced apart by an air zone ZA which allows the deformation or inclination of the contact probes 6 during the normal operation of the testing head, i.e. when such testing head contacts the device to be tested. Moreover, the upper 4 and lower 5 guide holes are sized so as to guide the contact probe 6.
FIG. 1 schematically shows a testing head 1 with non locked probes associated to a micro contact structure or space transformer, schematically indicated with 9A.
In this case, the contact probes 6 have a further contact tip 7A towards a plurality of contact pads 8A of the space transformer 9A, the good electrical contact between probes 6 and space transformer 9A being ensured similarly to the contact with the device to be tested by the pressure of the probes 6 on the contact pads 8A of the space transformer 9A.
A critical parameter in making a testing head is the distance (the so-called pitch) between the centers of the contact pads provided on the device to be tested. The pitch of integrated electronic devices, in fact, with the advancement of the corresponding manufacturing technologies, has become increasingly smaller, forcing to a high packing of the contact probes in the testing head, and causing positioning problems when one wants to avoid the mutual contact between the probes.
A distribution of contact pads on two sides is for example schematically shown in FIG. 2A. In the most recent technologies, the distance between the centers of the contact pads on the device to be tested, i.e. the pitch P indicated in the figure, has decreased down to values in the range between 30 μm and 80 μm. This reduction in the pitch mostly affects pad configurations on four sides, as shown in FIG. 2B. In this case, the distances between the contact centers of pads on a same row or on a same column, still called pitches and indicated in the figure as P1 and P2, respectively, have decreased down to values in the range between 30 μm and 80 μm.
The same problems, even bigger, are found in the testing of devices having pad configurations on four sides and multiple rows, as schematically shown in FIG. 2C.
Moreover, it should be noted that the testing may also involve multiple devices in parallel, as shown in FIG. 2D, the devices being separated from each other by so-called scribe lines SL, the thickness whereof, always lowering, complicates the testing operations of the pads of devices in parallel facing such scribe lines SL.
As already said, the decreasing the pitch value of the configurations of contact pads of newer devices to be tested, as well as the need of testing multiple devices in parallel introduces problems tied to the contact between adjacent probes.
In the case of contact probes of the type shown in FIG. 1, it is immediate to see that the minimum pitch substantially depends on the dimensions of the same probes 6. In the current vertical technologies, normally with probes having a circular cross-section, a reduction of the pitch value is thus obtained by reducing the diameter of the probes 6.
It is also known, for example from the European patent No. EP 1 243 931 granted on Jul. 17, 2004 to the Applicant, to offset the contact between the probe tip and the corresponding pads with respect to a longitudinal axis of the same probes so as to reduce the minimum pitch value.
In particular, the contact probes are provided with a stiff arm laterally projecting and extending in a direction being perpendicular or sloping by a suitable angle with respect to the probe. In other words, the stiff arm has a longitudinal axis which is perpendicular or sloped with respect to the longitudinal axis of the respective probe.
As schematically shown in FIG. 3B in a top view, a contact probe 15 comprises a stiff arm 12 laterally projecting from a body 11 of the probe 15 and ending with a contact tip 13 for contact pads 14 of a device to be tested.
As described in the patent No. EP 1 243 931 as above indicated, the offset between the longitudinal axis of the probes and the respective contact tips along with a suitable probe orientation allow the same probes to be placed in an alternatively opposed position, with respect to the contact pads, thereby considerably increasing the space available for the probe bodies, with a consequent remarkable reduction of the risks of a contact also for very small pitches, as schematically shown in FIG. 3A for a testing head 10 comprising a plurality of contact probes 15 of the type shown in FIG. 3B.
FIG. 4A shows an arrangement of the contact probes 15 particularly suitable for the case of contact pads 14 arranged on all the four sides of the device to be tested.
In particular, the adjacent probes 15 are arranged in an alternatively opposed position with respect to the contact pads 14 with symmetry axes which slope with respect to an alignment axis of the contact pads 14, forming a predetermined angle with this axis, in one preferred embodiment equal to 45°.
In the case shown, the contact probes 15 further have a projecting arm 12 arranged in offset position with respect to the probe body 11. In this case, left probes 15sx are those having an arm placed against a left side of the probe body 11 and right probes 15dx are those having an arm placed against a right side of the probe body 11, as shown in FIG. 4B.
The relative terms “right” and “left” are usual in the field and in the present description are used in relation to a local reference system of the figures.
In a preferred embodiment described in the above-indicated patent, the contact probes 15 have a non-circular cross-section, in particular a rectangular one. In this case, also the corresponding guide holes have a rectangular cross-section and the probes 15 inserted therein are always properly positioned with respect to the contact pads 14 of the device to be tested.
As indicated in the description text of the patent No. EP 1 243 931 in re, the probes with a rectangular cross-section, as shown by way of an example in FIGS. 3A, 3B, 4A and 4B, allow a further reduction of the used space when compared to the probes with a circular section and thereby a further reduction of the minimum pitch value.
In this case, the probe orientation and the exact position of the contact tip 13 on the contact pad 14 is ensured by the precise orientation of the guide hole having a rectangular cross-section that does not allow a rotation of probe 15 also having a rectangular section and being inserted therein.
While being advantageous from several viewpoints, the known solutions as those described in the European patent No. EP 1 243 931 as above indicated in any case have a limitation imposed by the overall dimensions of the probe body for the minimum pitch of the contact pads of the devices to be tested, especially in the case of a parallel testing and a testing of devices having pads on multiple rows.